Recently, with an increase in transmission traffic, development of an optical transmission device using a wavelength division multiplexing (WDM) system which enables large-capacity data communication has been advanced. By multiplexing signal lights having mutually different wavelengths, the optical transmission device using the WDM system simultaneously transfers different information. At this time, it is desirable that powers of the signal lights having mutually different wavelengths be adjusted uniformly to maintain transmission characteristics of the signal lights. Because of this, a technique which detects powers of signal lights having mutually different wavelengths is often used as a technique which becomes a prerequisite for uniformly adjusting the powers of the signal lights having mutually different wavelengths. For example, a photodetection device which detects powers of signal lights having mutually different wavelengths is used in the optical transmission device using the WDM system.
FIG. 6 illustrates a configuration example of a conventional optical transmission device equipped with photodetection devices which detect powers of signal lights having mutually different wavelengths in this way. FIG. 6 is a diagram illustrating the configuration example of the conventional optical transmission device. The conventional optical transmission device illustrated in FIG. 6 has laser diodes (hereinafter abbreviated as “LD”) 11a to 11n serving as light sources and photo detectors (hereinafter abbreviated as “PD”) 12a to 12n serving as light reception units.
The LDs 11a to 11n are respectively aligned corresponding to wavelengths “λ1 to λn” and emit signal lights having mutually different wavelengths in a forward direction. The PDs 12a to 12n are disposed behind the LDs 11a to 11n in a state of facing the LDs 11a to 11n and receive backlights serving as lights emitted backward from the respective LDs. Then, the conventional optical transmission device detects power of the signal light by multiplying a value which indicates an electric signal obtained by receiving the backlight by each of the PDs 12a to 12n and a power ratio of the signal light to the backlight stored in a predetermined storage unit.
Here, in the conventional optical transmission device, not only the backlight emitted from the LD which faces each PD but also a backlight emitted from the LD adjacent to the LD which faces the PD may be received by each PD. In other words, with recent miniaturization of a device, a structure in which a plurality of LDs is aligned in such a manner that an interval between adjacent LDs becomes narrow is often adopted. Due to this structure, the backlight emitted from the LD corresponding to a certain wavelength may be leaked into a PD side which faces an adjacent LD. Accordingly, each PD receives the backlight emitted from the LD adjacent to the LD which faces itself, and as a result, detection accuracy of the power of the signal light is deteriorated. For example, in the example illustrated in FIG. 6, in a case where the backlight emitted from the LD 11a corresponding to the wavelength “λ1” is leaked into the PD 12b which faces the adjacent LD 11b, a value of the power of the signal light detected by using the PD 12b becomes larger than an actual value.
On the other hand, a structure, in which a bandpass filter has a wavelength band corresponding to a backlight emitted from an LD as a transmission band and is provided on a front surface of each PD, has been proposed. In this structure, a light of a wavelength band other than the transmission band is cut off by the bandpass filter, and each PD receives only the backlight emitted from the LD which faces itself. With this configuration, detection accuracy of power of a signal light improves.
Patent Literature 1: Japanese Laid-open Patent Publication No. 06-152069
However, though the detection accuracy of the power of the signal light can be improved in the conventional technique where the bandpass filter is provided on the front surface of each PD, there is a problem such that it is difficult to realize miniaturization of a device.
In other words, in the conventional technique, since the bandpass filters are provided on the front surfaces of all of the plurality of PDs, the number of parts is increased by that of the bandpass filters, and as a result, a size of the device can be increased.